Revert r333226 "[ValueTracking] Teach computeKnownBits that the result of an...

Revert r333226 "[ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive."

This breaks some libFuzzer tests. http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer/builds/15589/steps/check-fuzzer/logs/stdio

Reverting to investigate

llvm-svn: 333253